Iranian hackers are developing software attacks that render computer systems inoperable until a digital ransom is paid, a new report says, a threat that comes as the U.S. moves to reimpose tough economic sanctions on the country.

Over the past two years, researchers at Accenture PLC’s iDefense cybersecurity-intelligence group have tracked five new types of so-called ransomware they say were built by hackers in Iran. The ransomware appears to be an attempt to secure payments in digital currencies such as bitcoin, Jim Guinn,...